The present disclosure relates generally to a fuel cell stack and assembly method thereof, and more particularly to the fuel cell stack and assembly method thereof by the use of an improved compression retention system during fuel cell stack fabrication.
Fuel cells convert a fuel into usable electricity via chemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™). The chemical reaction occurs when a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected in series along a common stacking dimension—much like a deck of cards—to form a fuel cell stack.
Such stacks are ordinarily assembled under a compressive load in order to seal the fuel cells and to secure and maintain a low interfacial electrical contact resistance between the reactant plates, the gas diffusion media and the catalyst electrodes that make up each cell. In one form, for a notional cell surface area of about 100 in2, a desired compression load on the fuel cell stack typically ranges from about 80 to 160 psi (i.e., about 40 to 80 kN), depending on humidification, and is maintained by a compression retention enclosure that acts as a housing around the fuel cell stack.
To establish the desired compressive force, the fuel cell stack is placed in a press that imparts a load to the stack, after which a compression retention system that includes the aforementioned enclosure is engaged, the press is released, and the stack is held under a pressure retained by the engaged compression retention system. In some cases, the compression retention system may thereafter be placed into a separate enclosure for environmental sealing, while in others, enclosing side panels and end plates may provide any necessary housing and sealing functions. In either case, the enclosed fuel cell stack is then mechanically and electrically secured to the vehicle or related device.
In the aforementioned design where the housing is defined by a series of side panels and end plates, it also typically includes interconnecting tie rods or bracketing elements to bind these discrete components, as well as to maintain a compressive force on the stacked fuel cells. The end plates are then compressed together by the brackets or tie rods that are mounted along the surface of one or more of the side panels. Compressive force is retained by securing the tie rods with bolts or related fasteners that extend normal to the generally planar surface of the side panels such that the bolts are loaded in shear.
Integration of fuel cell stacks into automotive platforms is a demanding challenge to the fuel cell system designer, necessitating precise placement and alignment with balance of plant (BOP) equipment that is situated inside of the vehicle's fuel cell system-receiving compartment. In the present context, BOP refers to components present in the vehicle, including but not limited to blowers, pumps, hoses, compressors or the like that are necessary for fuel cell stack integration, mounting and operability, but which are not part of the fuel cell system itself. Such integration demands tight dimensional tolerances of the assembled fuel cell stack.
Assembled height variance along the stacking direction (which may be thought of as the Z-axis in a conventional Cartesean coordinate system) may be as much as 5 to 10 percent of the overall stack height; the present inventors have noticed variances of up to about ±8 millimeters (or 16 millimeters total) along the stacking direction. Such variances in stack height make it difficult to design a stack possessive of consistent, repeatable dimensions. This in turn hampers subsequent BOP connection and overall system placement within the corresponding vehicle compartment. As such, it would be advantageous to provide a compression retention system which keeps fuel cell stack height variation within tightly-controlled limits. It would also be advantageous to compress a stacked series of fuel cells in such a way that adjustments to the stack height may be made while maintaining the stack in its compressed state in order to substantially achieve the desired height dimension of the assembled stack.